INSTANCES OF SUCCESSFUL HYBRIDS. 589'crossed with pollen of their own kind. If no insects appear upon the scene, auto¬gamy takes place. In either case fertile seeds are developed, and give rise to plantswhich do not differ from the parent-form. In places where detritus from theadjacent limestone and dolomitic cliffs is mixed with the humus of the soU theseedlings thrive distinctly better than those of R. ferrugineum and no less wellthan those of R. hirsutum. Thus, so far as the soil is concerned, R. intermediumhas an advantage over R. ferrugineum, and in respect of insect-visits it is betteroff than either of the parent-species. These advantages, though apparently insig¬nificant, are not merely sufficient to prevent R. intermedium, from yielding to theparent-species in the struggle for existence at the places in question, but even giveit rather a better chance of prevaiUng. As a second instance, we will take Salvia sylvestris, the hybrid offspring ofSalvia nemorosa and Salvia pratensis. This plant grows in dry meadows all overthe low country to the south of Vienna, as, for instance, on the banks of the riverLeitha, which separates Austria proper from Hungary. The landscape there isslightly undulating, the elevations are composed of boulders and clay, and whereverthe latter is present in great quantities, especially on the gentle slopes of the risingground. Salvia nemorosa constitutes an important item in the vegetation. Thehollows are full of a dark moist earth, with a rich admixture of humus, and therewe find meadows in which Salvia pratensis grows luxuriantly. These two kindsof habitat usually pass quite gradually into one another, and the parts common toboth consist of dry meadow-lands. Salvia nemorosa does not thrive in the intri¬cate grass-carpet of these meadows, and is rarely to be seen there, whilst for Salviapratensis the soil is too dry, so that that species also is but poorly represented onthe areas in question. On the other hand, these dry meadows are the most suitableground for the hybrid Salvia sylvestris, and it thrives exceedingly upon them.Its fiowers are much visited by insects; its fruits ripen in as large numbers asin the case of >Si. nemorosa or 8. pratensis, and have been found by experiment tobe fertile in a proportion of more than 60 per cent. Salvia sylvestris hastherefore scattered itself all over this dry meadow-land, and manifests all thecharacteristics essential to our conception of a species. A third example is Nuphar intermedium, a hybrid derived from Nupharluteum and Nuphar pumilum. This plant grows in lakes in the Black Forestand in the Vosges. It is also scattered over North Germany, and occurs withincreasing frequency in Central and Northern Russia and in Sweden. It has beenfound as far north as Lapland. At the northern extremity of this large area ofdistribution Nuphar intermedium is more abundant than the species from whichit is derived; indeed in many places it occurs in their absence, and in fact passesbeyond the northern limits of their area of distribution. In these situations thereis, of course, no possibility of the hybrid's crossing with either of the parent-species or of the formation of goneoclinic hybrids. Nuphar intermedium subsistsindependently there, multiplies without change of form, and has in fact establisheditself as a species. This phenomenon is explained as follows: The northern limit of